High-strength hot-dip galvanized steel sheet having excellent surface quality and electric resistance spot weldability, and manufacturing method therefor

Abstract

A hot-dip galvanized steel sheet according to one aspect of the present invention comprises a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the difference between the average of the Mn/Si values of surface oxides present on a surface portion, which is the region from the interface between the hot-dip galvanized layer and the base steel sheet to a depth of 15 nm, and the average of the Mn/Si values of internal oxides, which are present in the region from the interface to a depth of 50-100 nm, can be 0.5 or more. Mn and Si of each oxide mean the amounts (wt %) of Mn and Si components in the oxide, which are measured by EDS, and the average of Mn/Si values means the averaged value of the Mn/Si values measured for each oxide.

1 . A method for manufacturing a hot-dip galvanized steel sheet, comprising:
 providing a steel slab;   reheating the slab to a temperature of 950 to 1300° C.;   obtaining a steel sheet by hot rolling the reheated slab to a finish rolling start temperature of 900 to 1,150° C. and a finish rolling end temperature of 850 to 1,050° C.;   coiling the steel sheet in a temperature range of 590 to 750° C.;   pickling the steel sheet at a running speed of 180 to 250 mpm;   cold rolling the steel sheet at a reduction ratio of 35 to 60%;   recrystallization annealing the cold-rolled steel sheet by a process of heating the cold-rolled steel sheet under at conditions of moist nitrogen containing 5 to 10 vol % of H 2  as an atmospheric gas and a temperature and a dew point temperature at soaking zone being 650 to 900° C. and −10 to +30° C., respectively and cooling the cold-rolled steel sheet at a cooling rate of 5 to 30° C./s in a quenching zone; and   hot-dip plating the steel sheet by immersing the steel sheet in a hot-dip plating bath at a lead in temperature of 420 to 500° C.   
     
     
         2 . The method of  claim 1 , further comprising:
 alloying the hot-dip galvanized steel sheet at a temperature of 480 to 560° C.   
     
     
         3 . The method of  claim 2 , wherein the hot-dip plating bath contains one selected from the groups of 0.10 to 0.15 wt % of Al; 0.2 to 0.25 wt % of Al; and 0.7 to 5.7 wt % of Al and 0.7 to 5.7 wt % of Mg. 
     
     
         4 . The method of  claim 1 , wherein the running speed is 40 to 130 mpm during the recrystallization annealing. 
     
     
         5 . The method of  claim 1 , wherein the hydrogen concentration in the quench zone is 25 to 80 vol % during the recrystallization annealing. 
     
     
         6 . The method of  claim 1 , wherein the edge portion of the coiled steel sheet is heated at 600 to 800° C. for 5 to 24 hours. 
     
     
         7 . The method of  claim 2 , wherein the running speed is 40 to 130 mpm during the recrystallization annealing. 
     
     
         8 . The method of  claim 3 , wherein the running speed is 40 to 130 mpm during the recrystallization annealing. 
     
     
         9 . The method of  claim 2 , wherein the hydrogen concentration in the quench zone is 25 to 80 vol % during the recrystallization annealing. 
     
     
         10 . The method of  claim 3 , wherein the hydrogen concentration in the quench zone is 25 to 80 vol % during the recrystallization annealing. 
     
     
         11 . The method of  claim 2 , wherein the edge portion of the coiled steel sheet is heated at 600 to 800° C. for 5 to 24 hours. 
     
     
         12 . The method of  claim 3 , wherein the edge portion of the coiled steel sheet is heated at 600 to 800° C. for 5 to 24 hours.